Thermographs are employed for maintaining a thermal history of temperature sensitive goods. Temperature sensitive goods may include various foods, biochemicals, pharmaceuticals, biologicals, cultures, tissues, organs, and other thermolabile goods. Such goods typically have an optimal storage temperature of approximately 4.degree. C. However, such goods may undergo rapid degradation if they are exposed to temperatures outside of a specified range. For example, such goods may suffer freeze damage if their temperature drops below freezing; on the other hand, such goods may suffer denaturation, microbial attack, undesired enzymic reactions, or other effects at elevated temperatures, e.g. 20.degree. C. or higher.
Carmen G. Boeriu et al. (Bio/Technology, vol. 4, pp. 997-999 (November 1986)) disclose a thermographic composition which may be employed for monitoring frozen goods. Certain frozen goods suffer freeze-thaw damage if they are unintentionally thawed. Damage may occur even if the goods are promptly refrozen after the unintended thaw. Examples of such goods include frozen fish and various biochemicals. In order to monitor or detect the occurrence of an unintended thaw, Boeriu has developed a thermographic composition which undergoes a permanent color change above a specified temperature. The specified temperature may be 0.degree. C. or any other temperature within a wide range of 0.degree. C. The thermographic composition includes a paraffin which undergoes a solid-liquid transition at the specified temperature. An enzyme which drives a color reaction and its substrate are embedded within the solidified paraffin. Below the transition temperature of the solid paraffin, the enzyme and substrate are immobilized and inactive. Above the transition temperature, the enzyme becomes catalytically active and converts the substrate into its corresponding product. Typically the product will have a markedly different color than the substrate. When Boeriu's thermographic composition is stored in thermal contact with froozen goods which require monitoring, the occurrence of an unintended freeze-thaw cycle can be determined by the conversion of substrate to product, as determined by the appearance of the color reaction.
Boeriu's thermographic composition can also be employed for monitoring the thermal history of goods at temperatures other than the freezing temperature. However, Boeriu's thermographic composition can only detect uni-directional thermal transgressions, i.e. positive thermal transgressions which exceed a set temperature. Boeriu's thermographic composition can not monitor negative thermal transgressions, i.e. thermal transgressions which dip below a set temperature.
What is needed is a thermographic composition and device which is bi-directional, i.e. which can monitor both positive and negatitive thermal transgressions. Such a thermographic device would be useful for monitoring a thermal history within a set temperature range. Thermal transgressions which either exceeded or fell below the temperature range would be detected. Such a device would find utility monitoring chilled goods which should be maintained at 4.degree. C., which must not be allowed to freeze and which must not be exposed to elevated temperatures, except possibly for brief periods.
Additionally, it would be useful if the thermographic device provided a sharp transition to indicate thermal transgressions at the lower end of the temperature range, viz. to indicate an unintended freeze-thaw cycle. On the other hand, at the upper end of the temperature range, viz. for elevated temperatures, it would be useful if the thermographic device provided a ramp effect to indicate the magnitude of a thermal transgression. The ramp effect would intensify the color reaction to indicate thermal transgressions of greater duration and intensity.
Freeze damage typically occurs upon solidification and is not exacerbated by further exposure to deeply subfreezing temperatures. Hence, no ramp effect is required or desired on the lower end of the temperature range.
On the other hand, damage which occurs to goods upon exposure to elevated temperatures may increase with the magnitude of the transgression, i.e. damage arising from long exposures may exceed the damage arising from brief exposures and damage arising from exposure to very elevated temperatures may exceed the damage arising from temperatures which only slightly exceed the preferred temperature range. Thermographic compositions which have a ramp effect provide an indication of the duration and elevation of the thermal transgression. The ramp effect allows the thermographic composition to provide an approximate indication of the magnitude of the damage which occured to the goods being monitored. In effect, the ramp effect would provide an approximate integration of the duration and intensity of the thermal transgression.
The invention is a liposomal thermograph which employs a liposomal thermographic composition. A liposomal thermographic composition is a composition which is substantially non-fluorescent when maintained within a characteristic temperature range but which fluoresces brightly after having been exposed to temperatures outside of such characteristic temperature range. A brightly fluorescing liposomal thermograph is a signal that the goods with which the liposomal thermograph has been stored have been exposed to a temperature transgression, i.e. they have been exposured to elevated or depressed temperatures outside a characteristic temperature range. The liposomal thermographic composition includes liposomes which are loaded with a self-quenching fluorescent dye. Liposomes are vesicle structures having an outer lipid membrane and an interior region or lumen which is enclosed and defined by such outer membrane. The fluorescent dye is loaded into the lumen of the liposome. When a temperature transgression occurs, the fluorescent dye escapes from the lumen of the liposome. Upon its escape from the liposome, the fluorescent dye becomes diluted within the external solution and ceases to self-quench, i.e. the dye fluoresces brightly.
The liposomal thermographic composition of the present invention is bi-directional, i.e. it is useful for detecting both positive and negative temperature transgressions. This property is very useful when monitoring the thermal history of biological materials which must maintained in a chilled condition but which must not be allowed to freeze.